Case Study: APX

LOTUS APX VEHICLE

APX stands for “Aluminium Performance Crossover” and is the first example of a complete vehicle built on the innovative Lotus Versatile Vehicle Architecture (VVA) platform.

APX is a demonstration of Lotus Engineering’s ability to create innovative and exciting, high performance products through its world class capability in Vehicle Design, Performance Powertrain Engineering and niche vehicle development.

It is a 7-seater (a 5 +2 with the two rear seats being occasional) four-wheel drive “Crossover” vehicle with a front mounted 300 HP supercharged V6 petrol engine. Weighing in at just 1,570 kg and with a power to weight ratio of 191 HP per tonne, the APX has sportscar like performance of 5.4 seconds to 100 km/h (5.0 seconds to 60 mph) before reaching a top speed of 245 km/h (152 mph). These performance figures are as good as the highest performing 4x4 “Crossover” vehicles from other brands with up to 195 HP per tonne. Crucially though, whereas those vehicles need higher output engines to compensate for heavy weight, APX does not. Combined fuel consumption for APX is estimated to be 8.7 litres per 100 kilometres (32 mpg) – impressive on its own and more so when compared to its production rivals which often consume more than 13 litres per 100 kilometres (22 mpg).

APX is manufactured predominantly from aluminium in the form of high-pressure die-cast corner nodes, stampings and extrusions. It uses advanced assembly techniques, including adhesive bonding, self-piercing rivets and flow-drill screws for construction – joining techniques that Lotus calls Riv-Bonding. Lotus has optimised the use of these technologies thus significantly reducing the number of mechanical fixings within the monocoque structure. This has reduced the level of investment required in manufacturing equipment.

VEHICLE STRUCTURE

Prior to design release for the manufacture of this vehicle, the project was subject to industry standard engineering processes to ensure a quality product. Industry standard APQP processes have tracked and validated CAE (Computer Aided Engineering - Computer analysis for strength, durability, crash performance, Vibration, Aerodynamics, Fluid Flow etc), NCAP targets for crash, pedestrian impact, torsion, bending and modal stiffness targets, full static and dynamic CATIA DMU (Digital Mock Up - Digital build of the car to demonstrate the vehicle build and prove the fit of each part) to minimise build issues and the full suite of Material and Process simulation and validation to confirm manufacturing feasibility.

This coupled with tolerance analysis, full DFM/DFA (Design For Manufacture/Design For Assembly) and advanced joining technology research has been delivered in a true simultaneous engineering environment. The vehicle build proved very successful with only a handful of build concerns. A number of assembly validation builds were eliminated form the project saving both time and cost essential for niche vehicle projects needing ‘Right First Time’ design and engineering philosophies to overcome tight project constraints.

All significant components and structural items are made from Aluminium. This means that APX weighs in at a sector leading light weight of 1,570 kg so even though it is not a Lotus car, it adheres rigidly to the fundamental core values of the Lotus brand of “Performance Through Light Weight”. Of course lightweight structures are Lotus Engineering's forte and this is the industry recognised area where the greatest improvements in performance and fuel economy can be gained.

UNDERSTRUCTURE CONSTRUCTION

The understructure is riv-bonded aluminium, consisting of high-pressure die-castings, stampings and extrusions, and uses advanced assembly techniques, including adhesive bonding, self-piercing rivets and flow-drill screws for construction.The self-piercing rivets are used in a similar way to spot welding on a conventional steel shell, with the flow-drill screws used for single-sided access on closed sections. Both suffice to hold the structure together during the adhesive cure cycle, and contribute to the performance of the structure during both static and dynamic impact conditions. The heat-cured high strength structural adhesive is the main joining medium, and used in combination with the mechanical fasteners, produces an immensely strong, durable joint and a lightweight shell with exceptional torsional stiffness.

The whole vehicle is 4,697 mm long and 1,852 mm wide. Wheel base is 2,700 mm and track is 1,554 mm (front) and 1,556 mm (rear).

THE DESIGN

The brief for Lotus Design was to create a visually arresting “ Jekyll and Hyde “ car that fulfilled the needs of practical and flexible everyday transport but also provided the enthusiast with a focused driving machine. As with all crossover vehicles the solution is tailored to a specific niche buyer who will instantly identify with these unique attributes.

The flexibility of the VVA platform system enabled the Lotus Design Studio a rare opportunity to create a highly tailored solution that would meet, both, the complex requirements of the occupant and technical package as well as being aesthetically pleasing. The car has to look and feel like a sportscar but accommodate a 5+2 seating package.

INTERIOR DESIGN

The brief for the interior of the car was to create an environment with the visual drama of a sportscar but with a high degree of functionality and a contemporary sense of luxury.

The digital screens of the instrument pack have multi-functionality enabling major data, including Satellite Navigation, to be placed logically in front of the driver. Great care was taken in developing the grains, textures and material finishes to ensure an ambience of sportiness and contemporary luxury.

300HP 3 LITRE V6 ENGINE

The V6 engine has been designed and developed by Lotus Engineering’s powertrain division. The directive for the project was to produce a high performance prototype engine without the need to resort to exotic materials or manufacturing technology, allowing manufacture around the world.

APX’s engine is a supercharged 3 litre (2996 cc, Bore: 88 mm, stroke: 82.1 mm) V6 DOHC engine, mounted longitudinally in the front of the vehicle. It uses a Roots-type supercharger with a compact air – to – water Laminova – type charge cooler with separate parallel coolant system using its own water pump and heat exchangers allowing a charge temperature reduction of approximately 50 degrees Celcius.

Performance of the engine is maximum power of 224 kW (300 hp, 304 PS) at 6,250 rpm and a torque of 360 Nm at 4,500 rpm. 80% of peak torque is between 1,500 rpm and maximum revs of 6,500 rpm.

Like all products from Lotus, it follows the adage of “performance through light weight”, in that the engine weighs just 171 kg (fully dressed, dry weight). This gives a phenomenal specific output of around 1.31 kW/kg (1.7 hp/kg). Specific performance is 74.6 kW / litre (100 hp / litre, 101.3 PS / litre).

This light weight comes from using cast lightweight aluminium alloy cylinder block (with thin wall iron liners), aluminium cylinder head, and an aluminium structural oilpan, able to support the front drive shafts. The oilpan also includes coolant and lubrication channels to the oil cooler and filter, and an electronic oil level and temperature sensor. Even the pistons are lightweight – with each on weighing just 454 grams!

The construction of the V6 engine is arranged around the two banks of cylinders at an angle of 75 degrees with a compact cylinder offset of just 32.5 mm. This allows the engine to be compact, with good vibration characteristics, eliminating any requirement for balance shafts and the related extra cost, weight, height and complexity.

The Engine Management System uses a state of the art 32 bit ECU processor capable of over 200 million calculations per second. The ECU is Euro IV / E-OBD and LEV/OBDII compliant.

The engine satisfies the European End of Life Requirements (ELV) with respect to recyclability and recovery.

The V6 engine is an example of extensive research into “engine downsizing by Lotus Engineering’s Powertrain Team. Engine downsizing is the concept of using advanced technologies to enable a small engine to produce the power and torque of a much larger engine, with the obvious reduction in fuel consumption and emissions. One of the key demands of the automotive industry is to produce engines with not only low emissions and high fuel economy but also extremely good performance. These two demands for performance and economy are not normally compatible unless there is a significant change in the engine development strategy.